In recent years, advances in technology, as well as ever-evolving tastes in style, have led to substantial changes in the design of automobiles. One of the changes involves the complexity, as well as the power usage, of the various electrical systems within automobiles, particularly alternative fuel vehicles, such as hybrid, electric, and fuel cell vehicles.
Such vehicles often use electrochemical power sources, such as batteries, ultracapacitors, and fuel cells, to power the electric motors that drive the wheels, sometimes in addition to another power source, such as an internal combustion engine. An important parameter in the operation of vehicles that utilize batteries is the “state of charge” (SOC). The state of charge refers to the amount of stored energy in the battery that is available to be used at any given time relative to the amount of stored energy that is available when the battery is fully charged. An accurate determination of the state of charge allows for the vehicles to maximize performance and fuel economy or minimize emissions.
In automotive applications, a conventional approach for batteries is to relate either a measured or calculated open circuit voltage to the state of charge. This is feasible because the open circuit voltage, which is the resting voltage of the battery when no load is applied, generally exhibits some observable dependence on the battery's state of charge. There are batteries, however, such as nickel metal hydride and some types of lithium ion batteries, such as lithium iron phosphate batteries (e.g., nanophosphate lithium ion batteries), which possess a nearly constant open circuit voltage across most of the range of state of charge. In other words, the open circuit voltage reveals little about the state of charge of the battery. For example, in some nanophosphate lithium ion batteries, increases in the state of charge from 0% to 100% results in only a 7% change in the open circuit voltage.
Therefore, while these batteries are highly desirable as power sources for electric and hybrid vehicles because of their low mass, high power capability, and large energy storage capacity, they present a problem with regard to control because it is very difficult to estimate their state of charge with any degree of certainty.
Other techniques have also been used to determine the state of charge of batteries, such as ampere-hour (Ah) counting and electrochemical impedance spectroscopy (EIS). However, they too have drawbacks in automotive applications due to, for example, accuracy and/or high implementation costs.
Accordingly, it is desirable to provide a method and a system for determining the state of charge of a battery that is not based on, or solely based on, open circuit voltage or current flow. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.